1. Field of the Invention
The present invention relates to switching devices and related operating methods for turning switching power elements on and off and, more particularly, to a switching device and related operating method for performing switching control so as to suppress switching noises occurring during operations of a switching power element.
2. Description of the Related Art
Switching device of this type includes a switching device that is used in an onboard power electronics device installed on, for instance, a vehicle. With the vehicle installed with such an onboard power electronics device, noises occur during operations of the switching device and overlap a frequency of a broadcast station received by an on-vehicle radio receiver. With the frequency of the selected broadcast station overlapped with noises, a speaker of a car audio unit outputs an audible frequency (sound) to which noises are mixed making the user feel uncomfortable.
Technologies have been well known in the art wherein in order to suppress such noises, a spectrum spread scheme is employed to diffuse energies of harmonic components of a switching frequency with a view to eliminating an energy level of average noise. However, although such a technology makes it possible to reduce energies of noises caused by the respective switching frequency and associated harmonic components, difficulties are encountered in the related art in which when the switching frequency and associated harmonic components are overlapped with the frequency of the broadcast station, the speaker outputs noise in an uncomfortable fashion.
Further, although a noise filter is conceivably provided on the switching device at an output side thereof to achieve reduction in noise output from the speaker, the noise filter becomes complex in structure and large in size in order to adequately suppress noise from the speaker.
To address such an issue, attempts have heretofore been made to provide a switching device arranged to set a switching frequency of a switching power element such that harmonics of the switching frequency have a given relationship with a frequency band of a selected broadcast station as disclosed in Japanese Patent Laid-Open Publications No. 2002-335672 and No. 2003-88101. That is, with such a switching device, a train of drive pulses is generated such that the switching frequency is set to a specified frequency different from the selected broadcast station to avoid a speaker from outputting noise.
However, due to piece-piece variations of component parts forming the switching device, the switching frequency is liable to vary with the resultant difficulty in setting the switching frequency with a given relationship maintained with respect to the frequency band of the selected broadcast station. Also, in general practice, an AM broadcast station has one station with a frequency band (bandwidth) in a range of “9 kHz” in Japan and “10 kHz” in USA. Thus, it seems distant for the switching frequency to be set in the given relationship within such narrow frequency bands.